There has been great interest in hydrogen as an energy carrier in the transport sector. Hydrogen is perceived as environmentally friendly, in particular, because no CO2 emissions are caused at its combustion by the end consumer. This view, however, ignores that greenhouse gas emissions are caused during its production and supply. Since energy losses occur at every energy conversion, the hydrogen supply infrastructure is, at present, relatively inefficient due to its complexity. This constitutes an essential disadvantage, in particular, in the spreading of hydrogen technologies for use as fuels, either in fuel cells or in internal combustion engines, as a substitution for non-renewable fossil fuels.
Hydrogen, a colorless and odorless gas that is almost insoluble in water, was discovered by the English scientist Henry Cavendish in 1766. On a laboratory scale, it is produced by the electrolysis of water or by the exposure of zinc or iron to diluted acids. On an industrial scale, it is produced by a two-step method, with CO and H2 being produced in a first step by burning hydrocarbons with steam, and CO being converted to CO2 in a second step by the water gas reaction (CO+H2O→CO2+H2). Carbon dioxide is then eliminated by scrubbing.
According to estimates of IEA, 95% of today's worldwide hydrogen production originates from carbon-containing raw materials, mostly of fossil origin. Most of the conventional processes convert said carbon to CO2, which escapes into the atmosphere. The knowledge about the influence of greenhouse gases on global climatic changes has now called for a reassessment of these conventional approaches. It may also be anticipated that underground storage of collected CO2, to the extent coupled with conventional steam reforming processes, will not rapidly lead to absolutely clean production of hydrogen from fossil energy carriers.
There are basically two ways of producing hydrogen from hydrocarbons, namely oxidizing conversion and non-oxidizing conversion.
Steam-reforming of natural gas (methane, in the first place), also referred to as SMR, is a very highly developed commercial bulk process, by which 48% of the world's hydrogen production is accomplished. This technology is also feasible with other raw materials such as ethane or naphtha, yet its efficiency will be lower with such higher-molecular substances (C. E. Gregoir Padro and V. Putsche, “Survey of the Economics of Hydrogen Technologies”, September 1999, National Renewable Energy Laboratory). The SMR technology is based on the reaction of methane with steam in the presence of a catalyst. On an industrial scale, this process is run at about 800° C. and a pressure of 2.5 MPa. The first process stage comprises the conversion of methane with steam to CO and hydrogen. In the second stage, which is also referred to as water gas reaction, CO is further reacted with steam, thus yielding CO2 and additional hydrogen. With the aid of membranes, the occurring CO2 is separated from the product gas, which is free of other impurities in further process steps. The gas occurring in those steps, which, after all, consists of 60% combustible components, is recycled into the reformer.
With reference to the Kyoto Protocol and various national legislatures aimed at minimizing greenhouse gases, the greatest drawback of the SMR process is its high CO2 emission. To prevent this is the key issue of the present invention. Moreover, the process described herein encompasses the economic conversion of hydrocarbon to hydrogen gas and additionally exploitable fiber-shaped hydrocarbon (nanotubes).
Non-oxidizing methods include the thermal decomposition, also referred to as temperature-induced dissociation, the pyrolysis or the cracking of hydrocarbons to hydrogen and carbon.
The thermal decomposition of natural gas has been carried out for long and constitutes one of the most important processes for the production of carbon black. In this context, natural gas is decomposed at high temperatures ranging from 1200 to 1800° C. to form hydrogen and carbon black, wherein air, oxygen or steam are preferably admixed to both modify the carbon black formed and maintain the reactor temperatures. General literature on this topic can be taken from Monographie, Winnacker-Kuichler, Vol. 3, anorganische Technologie II, 4.sup.th Edition, Carl Hanser Verlag, 1983. A new development relating to the decomposition of methane was recently presented by the Norwegian enterprise Kvaemer, wherein hydrogen and carbon black are produced in a high-temperature plasma (CB&H Prozess, Proc. 12th World Hydrogen Energy Conference, Buenos Aires, 697, 1998). Advantages of that plasma-chemical process are its high thermal efficiency (>90%) and the purity of the produced hydrogen (98 vol. %). On the other hand, it is a very energy-intensive process.
In order to reduce the high reaction temperatures, catalyst-supported processes were proposed. There, it turned out those transition metals exhibited high activities in terms of methane decomposition, yet with the drawback of carbon layers depositing on the surfaces of the catalysts. In most cases, the thus formed carbon coat was burned off under air access in order to regenerate the catalyst, which, in turn, resulted in all of the carbon having been converted to CO2 and hydrogen having been the sole product to be utilized.
U.S. Pat. No. 1,868,921, Schmidt et al., reports on the conversion of unsaturated hydrocarbons, preferably ethylene, to carbon black at temperatures of about 600.degree. C. by the aid of nickel or cobalt catalysts applied on diatomaceous earth or ZnO, yet does not mention any appreciable synthesis of hydrogen. U.S. Pat. No. 2,760,847, Oblad et al., deals with the decomposition of low-molecular hydrocarbons for the production of hydrogen by contact reactions on transition metals of groups VI/b and VIII of the Periodic System, which are dispersed in liquid host metal phases. U.S. Pat. No. 3,284,161, Pohlenz et al., describes a process for continuously producing hydrogen by catalytically decomposing gaseous hydrocarbons. Methane is cracked in a catalyst fluidized bed at temperatures of between 815 and 1093° C. That process uses Ni, Fe and Co catalysts, preferably Ni/Al2O3, which are deposited on carriers. The catalyst coated with carbon is continuously removed from the reactor, and the carbon is burned in a regenerator, whereupon the recovered catalyst is recycled into the reactor.
Ermakova et al. examines the effect of the SiO2 content on Ni and Fe catalysts for the synthesis of carbon filaments, also proposing the efficiency of these catalysts for the preparation of hydrogen [Ermakova et al., Catalysis Today, 77, (2202), 225-235]. The authors report on Ni and Fe—SiO2 catalysts having metal contents of between 85 and 90 wt % and effectively decomposing methane into carbon filaments and hydrogen. The catalyst production comprises a two-stage process, wherein α-Ni(OH2) with a large specific surface area is dispersed into an SiO2-containing alcohol and the resulting mixture is calcined at temperatures of up to 700° C. Although the catalyst reduced at 700° C. had the smallest specific surface area (7 m2/g), it exhibited the highest catalytic activity. By way of comparison, the catalyst calcined at 250° C. according to BET had a specific surface area of 400 m2/g. Tests in which methane was catalytically decomposed revealed that methane can be decomposed by 16% with 10 mg catalyst. At a reaction temperature of 550° C. and a volume flow of 20 ml/min methane, the useful life of the catalyst was indicated with 30 hours. Various other catalysts are comprehensively known in the prior art.
In U.S. Pat. No. 6,315,977 B, a method for producing hydrogen and nanotubes is described, in which a hydrocarbon gas is reacted in a reactor including two different zones, said zones differing in terms of temperature and catalyst composition.
JP 2003-146606A describes a method for producing hydrogen, in which hydrocarbons on carbon nanohorns are decomposed to hydrogen and carbon. Such carbon nanohorns constitute alternative catalyst surfaces to metals.
From JP 2004-236377A, a water gas shift reaction catalyst can be taken, which is comprised of a titanium nanotube. Such a catalyst can be used for reducing NOx from exhaust gases.
Another catalyst for the production of hydrogen is described in JP 2004-074061A. That catalyst is comprised of a carrier based on silica titanium carbon fibers or carbon nano-fibers, which is impregnated with palladium and nickel compounds.
A copper catalyst with a nano-carbon material for the recovery of hydrogen from methanol can be taken from CN 1586718A.
EP 1623957A describes a method for producing hydrogen, by which also nano-carbon compounds occur, wherein an Ni catalyst is preferably used. There is a considerable number of publications dealing with the synthesis of graphite fibers of less than 1 μm diameter, yet far more than 1 μm length. At least some generally acceptable base facts have been established on this topic.
Suitable catalysts are the transition elements of group VIII of the Periodic System, Fe, Ni and Co, which, in the presence of carbon, are able to form Me3C cementit phases that are metastable in certain temperature ranges. Although there is some kinetic stability, such Me-C systems will only be in a thermodynamic equilibrium if metal and graphite are present as separate phases.
The carbon supplying species must form a stable vapor or gas phase at least in a defined time interval.
The diameter of the catalytically formed fibers or whiskers is directly related to the size of the catalyst particles.
Key technologies required for the breakthrough of hydrogen as a fuel, in addition to the production of hydrogen, also relate to its storage, transport and conversion into energy.
Hydrogen can be stored in large amounts only with energy expenditures, usually either as a gas or as a liquid. Gasometers are used for very large volumes. Medium quantities are stored as a gas in pressure tanks at about 30 bar. Smaller amounts can be filled into high-pressure gas bottles of steel or carbon-fiber-reinforced composite material at, presently, up to 400 bar. Yet, hydrogen can also be stored in liquid form at minus 253° C. All these types of storage involve considerable energy expenditures, both in terms of storage and in terms of maintaining, e.g., a cooled storage tank. The supply of a filling station network may finally be realized by the aid of tank trucks. If hydrogen is stored in high-pressure gas bottles of steel, only very little gas can be transported at a considerable weight. Thus, a 40-ton truck will only be able to transport about 530 kilograms of gaseous hydrogen in steel bottles. By contrast, the transport of deep-frozen liquefied hydrogen in extremely well-insulated, double-walled tank containers is economical even for large volumes. The same 40-ton truck, with the appropriate tank system, will be able to load around 3300 kilograms of liquid hydrogen. As in the case of storage, considerable energy losses will have to be taken into account during transportation. An essential prerequisite for the introduction of hydrogen as a fuel for vehicles is a production and distribution system that must not be more complicated than today's system.
In order to shorten transport paths, U.S. Pat. No. 6,432,283B1 proposes to produce hydrogen from water directly at filling stations through electrolysis. Due to the high energy consumption in the form of electric power during the electrolysis of water, this method is, however, neither economical nor ecological, considering that current is primarily obtained by the combustion of fossil fuels.
Another problem involved in the use of hydrogen as a fuel for motor vehicles is its low energy density as compared to gasoline or diesel. Fuel consumption will consequently be higher, and vehicles will have to be equipped with larger tanks. In order to adjust the energy content of fuel to the respective demand, superior fuels are sought, which are relatively environmentally compatible. Hythane-operated vehicles are regarded as an intermediate stage towards vehicles operated by pure hydrogen. Hythane is a mixture of hydrogen and methane with variable composition portions. From CA 2141065 and EP 0805780B1, a method for producing hythane from methane is known, which has a composition comprising a hydrogen portion of about 5-20%. In a pilot project performed with hythane-operated buses in Montreal, it could be demonstrated that hythane-operated buses were significantly more environmentally compatible than buses operated with natural gas (methane). By that choice of fuel, the NO emission, for instance, was reduced by about 50%. The portion of hydrogen in the fuel amounted to 20 vol % (or 6% of the energy portion) in that project. When classifying hythane, it is also to be considered that pure hydrogen will not burn NON-free per se. Due to the high combustion temperature of hydrogen at a comparable air/fuel ratio, higher NO emissions will actually occur by its utilization in an internal combustion engine (air oxygen). Optimally low NO emissions with the use of hythane can be achieved at a hydrogen portion of between 20 and 30%.
The establishment of a hydrogen-oriented infrastructure is an expensive project. Not only production costs will have to be lowered, but also transport and storage costs will have to be reduced. At present, hydrogen is not able to compete on the market with conventional hydrocarbon-based fuels. It is an object of the present invention to both accelerate this transition and enable the supply of motor vehicles with hydrogen at filling stations. It is a further object to provide a suitable reactor for the production of hydrogen, which can be used at filling stations in an economic manner.